On FreeBSD if you use the "-i" flag with sed you must specify a backup file. The simple fix, if you don't want a backup file is to use "" (two quotes, no space between them). You could also use any other filename you wished.
BAD: sed -i s/192.168.1.1/192.168.1.44/g file.txt
GOOD: sed -i "" s/192.168.1.1/192.168.1.44/g file.txt